A variety of approaches are known in the art for separating volatile and semi-volatile organic compounds, as well as for separating mercury from soil and similar solids. For example, various methods and apparatus for thermally separating volatile components are known. At least one objective of these methods has been the removal of contaminants such that resulting solids could be managed without regard to the former contamination. This objective has been accomplished using both low temperature, fixed processing vessels, and also using high temperature rotating processing vessels.
However, known approaches have limitations that become increasingly significant when the volume of material requiring treatment is too small to justify large costly treatment systems. Simply making the equipment smaller, to reduce initial cost, yields the equally limiting feature of low processing rate or capacity. Furthermore, certain desirable separation processes can occur only at higher temperatures. Thus, the low treatment temperatures imposed by certain separation vessels that are heated using heat transfer oils prevent achievement of the separation. Additionally, it is often prohibitively expensive to rapidly achieve low contaminant levels at the lower temperatures provided by these vessels heated by heat transfer oils.
U.S. Pat. Nos. 5,253,597 and 5,453,562 (Swanstrom et al.) describe a batch process that operates under strong vacuum with heat from a conventional hot oil heating system. This approach imposes an upper temperature limitation of 600 degrees Fahrenheit (° F.) for the treated solids. This limitation results in the inability to treat compounds that must be heated to significantly higher temperatures to undergo a chemical reaction prior to separation, such as thermal reduction of mercury salts to elemental mercury, depolymerization of organic plastics, or thermolysis of cellulose. Also, low temperature gradients result from this temperature limitation during portions of the treatment that require significantly longer treatment times and reduce both the capacity and economic viability of the process. For example, treatment times using a process described in U.S. Pat. No. 5,253,597 and U.S. Pat. No. 5,453,562 extend for several days due to low mass transfer rates when residual contaminant levels approach the part per million level, resulting in high operating cost and inefficient operations. This limitation is attributable to the restricted operating temperature imposed by the hot oil heating system.
U.S. Pat. Nos. 5,628,969 (Aulbaugh et al.) and 5,514,286 (Crosby) teach approaches that use a rotatable vessel with a fixed internal filter. These apparatus are mechanically complex and cannot be operated in a semi-continuous mode because of the complexities of introducing materials to and removing them from the rotating vessel (e.g., while maintaining the seal to achieve high vacuum).
U.S. Pat. No. 5,490,907 (Weinwurm et al.) discloses a method for treating sludges in which valuable liquids are recovered from the sludges. This method requires the addition of a reagent powder to the thermal processor to form a high surface area semi-solid. The heating vessel is restricted, however, to a maximum temperature of 350° C., thus imposing the same limitations as the processes of U.S. Pat. Nos. 5,253,597 and 5,453,562.
Other separation processes are disclosed in U.S. Pat. Nos. 4,864,942 (Fochtman et al.) and 4,402,274 (Meenan et al.), which involve the heating of organically contaminated solids in a continuous thermal unit with condensation and recovery of the contaminants. These processes are operated strictly on a continuous basis and require elaborate material feed and removal systems. Also, these processes allow a significant quantity of solids to migrate into the gas treatment system. Substantial equipment is thus needed to remove and manage these solids. This result is imposed by the nature of a typical continuous separation process. Additionally, the apparatus used in these continuous separation processes cannot be sealed, thus cannot operate at high vacuum.